Optical fiber illumination systems and methods

ABSTRACT

An illumination system generating light having at least one wavelength within 200 nm a plurality of nano-sized structures (e.g., voids). The optical fiber coupled to the light source. The light diffusing optical fiber has a core and a cladding. The plurality of nano-sized structures is situated either within said core or at a core-cladding boundary. The optical fiber also includes an outer surface. The optical fiber is configured to scatter guided light via the nano-sized structures away from the core and through the outer surface, to form a light-source fiber portion having a length that emits substantially uniform radiation over its length, said fiber having a scattering-in- duced attenuation greater than 50 dB/km for the wavelength(s) within 200 nm to 2000 nm range.

This application is a continuation of U.S. patent applica- tion Ser. No.12/950,045 filed on Nov. 19, 2010 entitled “Optical Fiber IlluminationSystems and Methods” claims the benefit of priority under 35 USC 119(e)of U.S. Provi- sional Application Ser. No. 61/263,023 filed Nov. 20,2009.

More than one reissue application has been filed for the reissue of U.S.Pat. No. 8,545,076. The reissue applications are U.S. patent applicationSer. No. 14/689,756, filed Apr. 17, 2015, and the present application.

The present application is a continuation reissue application of U.S.patent application Ser. No. 14/689,756, filed Apr. 17, 2015, now U.S.Pat. No. RE46,098, which is an application for reissue of U.S. Pat. No.8,545,076, issued Oct. 1, 2013, from U.S. patent application Ser. No.13/213,363, filed Aug. 19, 2011, which is a continuation of U.S. patentapplication Ser. No. 12/950,045, filed on Nov. 19, 2010, now U.S. Pat.No. 8,591,087, which claims the benefit of priority under 35 U.S.C.119(e) of U.S. Provisional Application Ser. No. 61/263,023, filed Nov.20, 2009.

BACKGROUND

1. Field

The present invention relates generally to light diffusing opticalfibers having a region with nano-sized structures, and in particular toillumination systems and methods that employ such fibers for differentapplications including bioreactors, signage and special lightingapplications.

2. Technical Background

Optical fibers are used for a variety of applications where light needsto be delivered from a light source to a remote location. Opticaltelecommunication systems, for example, rely on a network of opticalfibers to transmit light from a service provider to system end-users.

Telecommunication optical fibers are designed to operate atnear-infrared wavelengths in the range from 800 nm to 1675 nm wherethere are only relatively low levels of attenuation due to absorptionand scattering. This allows most of the light injected into one end ofthe fiber to exit the opposite end of the fiber with only insubstantialamounts exiting peripherally through the sides of the fiber.

Recently, there has been a growing need to have optical fibers that areless sensitive to bending than conventional fibers. This is because moreand more telecommunication systems are being deployed in configurationsthat require the optical fiber to be bent tightly. This need has lead tothe development of optical fibers that utilize a ring of smallnon-periodically disposed voids that surround the core region. The voidcontaining ring serves to increase the bend insensitivity—that is tosay, the fiber can have a smaller bend radius without suffering asignificant change in the attenuation of the optical signal passingthrough. In these fibers the void containing ring region is placed inthe cladding of the optical fiber some distance from the core in orderto minimize amount of the light propagation through void containing ringregion, since it could increase optical loss.

Because optical fibers are typically designed to efficiently deliverlight from one end of the fiber to the other end of the fiber over longdistances, very little light escapes from the sides of the typicalfiber, and, therefore optical fibers are not considered to bewell-suited for use in forming an extended illumination source. Yet,there are a number of applications such as special lighting, signage, orbiological applications, including bacteria growth and the production ofphoto-bioenergy and biomass fuels, where select amounts of light need tobe provided in an efficient manner to the specified areas. For biomassgrowth there is a need to develop processes that convert light energyinto biomass-based fuels. For special lighting the light source needs tobe thin, flexible, and easily modified to variety of different shapes.

SUMMARY OF THE INVENTION

According to some embodiments, first aspect of the invention is anillumination system that generates light having at least one wavelength(λ) within the 200 nm to 2000 nm range. The system includes a lightsource and at least one light diffusing optical fiber. The lightdiffusing optical fiber has a core and a cladding. A plurality ofnano-sized structures is situated either within said core or at acore-cladding boundary. The optical fiber also includes an outersurface, and an end optically coupled to the light source. The opticalfiber is configured to scatter guided light via said nano-sizedstructures away from the core and through the outer surface, to form alight-source fiber portion having a length that emits substantiallyuniform radiation over its length, said fiber having ascattering-induced attenuation of greater than 50 dB/km for saidwavelength. According to some embodiments the light source coupled tothe fiber generates light in 200 nm to 500 nm wavelength range andfluorescent material in the fiber coating generates either white, green,red, or NIR (near infrared) light.

According to some embodiments, the illumination system includes a singlelight diffusing fiber. According to other embodiments the illuminationsystem includes a plurality of light diffusing fibers. These lightdiffusing fibers may be utilized in a straight configuration, or may bebent.

According to some exemplary embodiments the illumination system may beused for a biological growth system, and may further include abiological chamber with an interior configured to contain biologicalmaterial. In these embodiments a light source generates light having awavelength to which the biological material is sensitive. The fiber mayhave a plurality of bends formed therein so as to scatter guided lightaway from the central axis, out of the core, and through the outersurface to form a light-source fiber portion having a length that emitssubstantially uniform radiation over its length.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

Thus, one advantageous feature of at least some embodiments of thepresent invention is that the illumination systems and methods thatutilise the optical fibers' ability to efficiently deliver light toremote locations and that such fibers will scatter light relativelyuniformly even when deployed in different shapes (e.g., bent, coiled orstraight) to match the needs of application. In addition, oneadvantageous feature of some embodiments of the present invention isthat the light diffusing fibers according to at least some of theexemplary embodiments of the present invention are capable of providingillumination with a weak wavelength dependence, wherein the scatteringloss L_(S) within the 200 nm to 2000 nm wavelength range is proportionalto λ^(−p), where p is greater or equal to 0 and less than 2, preferablyless than 1, and more preferably less than 0.5 or even more preferablyless than 0.3. Another advantageous feature of at least some embodimentsof the present invention is the capability of having substantiallyuniform scattering along the length (e.g., less than 50% and morepreferably less than 30% variation from the maximum), and in angularspace away from the axis of the fiber, such that forward, 90 degrees(from axis of the fiber) and backward scattering intensities are almostthe same (e.g., within 30% of each other, and preferably within 20% ofeach other).

In at least some embodiments, the variation of the integrated lightintensity coming through sides of the optical fiber (i.e., the intensityvariation of the diffused or scattered light) at the illuminationwavelength is less than 30% for the target length of the optical fiber.

In at least some embodiments, the average scattering loss of the fiberis greater than 50 dB/km, and the scattering loss does not vary morethan 30% (i.e., the scattering loss is within ±30% of the averagescattering loss) over any given fiber segment of 0.2 m length. Accordingto at least some embodiments, the average scattering loss of the fiberis greater than 50 dB/km, and the scattering loss does not vary morethan 30% over the fiber segments of less than 0.05 m length. Accordingto at least some embodiments, the average scattering loss of the fiberis greater than 50 dB/km, and the scattering loss does not vary morethan 30% (i.e., ±30%) over the fiber segments 0.01 m length.

One advantageous feature of the fibers according to some embodiments ofthe present invention and of the illumination system utilising suchfibers, is that the fiber acts as a light source, and illuminates thedesired medium by uniformly scattering light through the sides of thefiber rather than delivering an intense and localized beam of light fromthe end of the fiber. Furthermore, in some embodiments, the use of fiberadvantageously allows the electrically driven light source to remaindistant from the point(s) of light delivery. This fact would be mostbeneficial in aqueous or potentially explosive environments where theelectrical components could be located a safe distance from theconductive or reactive environment.

It is to be understood that both the foregoing general description andthe following detailed description represent embodiments of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated into and constitutea part of this specification. The drawings illustrate variousembodiments of the invention and together with the description serve toexplain the principles and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a section of an example embodiment oflight-diffusing optical fiber;

FIG. 2 is a schematic cross-section of the optical fiber of FIG. 1 asviewed along the direction 2-2;

FIG. 3A is a schematic illustration of relative refractive index plotversus fiber radius for an exemplary embodiment of light diffusingfiber;

FIG. 3B is a schematic illustration of relative refractive index plotversus fiber radius for another exemplary embodiment of light diffusingfiber;

FIG. 3C is illustrates another exemplary embodiment of a light diffusingfiber;

FIGS. 4A and 4B depict fiber attenuation (loss) in dB/m versuswavelength (nm);

FIG. 5 illustrates a fiber deployment that utilizes two light passeswithin a single fiber;

FIG. 6A illustrates the intensity distribution along the fiber when thefiber made with uniform tension (example A) and variable tension(example B);

FIG. 6B illustrates the scattering distribution function with white inkand without ink;

FIG. 7 illustrates scattering for fiber shown in FIG. 5 (with reflectivemirror at coupled to the rear end of the fiber), and also for a fiberutilizing white ink in its coating;

FIG. 8A illustrates an exemplary embodiment of illumination system;

FIG. 8B illustrates an example embodiment of illumination system as usedin combination with a biological chamber in the form of a flask;

Additional features and advantages of the invention will be set forth inthe detailed description which follows and will be apparent to thoseskilled in the art from the description or recognized by practicing theinvention as described in the following description together with theclaims and appended drawings.

DETAILED DESCRIPTION

Reference is now made in detail to the present preferred embodiments ofthe invention, examples of which are illustrated in the accompanyingdrawings. Whenever possible, like or similar reference numerals are usedthroughout the drawings to refer to like or similar parts. It should beunderstood that the embodiments disclosed herein are merely examples,each incorporating certain benefits of the present invention.

Various modifications and alterations may be made to the followingexamples within the scope of the present invention, and aspects of thedifferent examples may be mixed in different ways to achieve yet furtherexamples. Accordingly, the true scope of the invention is to beunderstood from the entirety of the present disclosure, in view of butnot limited to the embodiments described herein.

DEFINITIONS

Terms such as “horizontal,” “vertical,” “front,” “back,” etc., and theuse of Cartesian Coordinates are for the sake of reference in thedrawings and for ease of description and are not intended to be strictlylimiting either in the description or in the claims as to an absoluteorientation and/or direction.

In the description of the invention below, the following terms andphrases are used in connection to light diffusing fibers havingnano-sized structures.

The “refractive index profile” is the relationship between therefractive index or the relative refractive index and the waveguide(fiber) radius.

The “relative refractive index percent” is defined asΔ(r) %=100×[n(r)²−n_(REF) ²)]2n(r)²,where n(r) is the refractive index at radius r, unless otherwisespecified. The relative refractive index percent is defined at 850 nmunless otherwise specified. In one aspect, the reference index n_(REF)is silica glass with the refractive index of 1.452498 at 850 nm, inanother aspect is the maximum refractive index of the cladding glass at850 nm As used herein, the relative refractive index is represented by Δand its values are given in units of “%”, unless otherwise specified. Incases where the refractive index of a region is less than the referenceindex n_(REF), the relative index percent is negative and is referred toas having a depressed region or depressed-index, and the minimumrelative refractive index is calculated at the point at which therelative index is most negative unless otherwise specified. In caseswhere the refractive index of a region is greater than the referenceindex n_(REF), the relative index percent is positive and the region canbe said to be raised or to have a positive index.

An “updopant” is herein considered to be a dopant which has a propensityto raise the refractive index relative to pure undoped SiO₂. A“downdopant” is herein considered to be a dopant which has a propensityto lower the refractive index relative to pure undoped SiO₂. An updopantmay be present in a region of an optical fiber having a negativerelative refractive index when accompanied by one or more other dopantswhich are not updopants. Likewise, one or more other dopants which arenot updopants may be present in a region of an optical fiber having apositive relative refractive index. A downdopant may be present in aregion of an optical fiber having a positive relative refractive indexwhen accompanied by one or more other dopants which are not downdopants.

Likewise, one or more other dopants which are not downdopants may bepresent in a region of an optical fiber having a negative relativerefractive index.

The term “α-profile” or “alpha profile” refers to a relative refractiveindex profile, expressed in terms of Δ(r) which is in units of “%”,where r is radius, which follows the equation,Δ(r)=Δ(r_(o))(1−[|r−r_(o)↑/(r₁−r_(o))]^(α)),where r_(o), is the point at which Δ(r) is maximum, r, is the point atwhich Δ(r) % is zero, and r is in the range r_(i)≤r≤r_(f), where Δ isdefined above, r, is the initial point of the α-profile, r_(f) is thefinal point of the α-profile, and a is an exponent which is a realnumber.

As used herein, the term “parabolic” therefore includes substantiallyparabolically shaped refractive index profiles which may vary slightlyfrom an α value of 2.0 at one or more points in the core, as well asprofiles with minor variations and/or a centerline dip. In someexemplary embodiments, α is greater than 1.5 and less than 2.5, morepreferably greater than 1.7 and 2.3 and even more preferably between 1.8and 2.3 as measured at 850 nm In other embodiments, one or more segmentsof the refractive index profile have a substantially step index shapewith an a value greater than 8, more preferably greater than 10 evenmore preferably greater than 20 as measured at 850 nm.

The term “nano-structured fiber region” describes the fiber having aregion or area with a large number (greater than 50) of gas filledvoids, or other nano-sized structures, e.g., more than 50, more than100, or more than 200 voids in the cross-section of the fiber. The gasfilled voids may contain, for example, SO₂, Kr, Ar, CO₂, N₂, O₂, ormixture thereof The cross-sectional size (e.g., diameter) of nano-sizedstructures (e.g., voids) as described herein may vary from 10 nm to 1 μm(for example, 50 nm-500 nm), and the length may vary from 1 millimeter50 meters (e.g., 2 mm to 5 meters, or 5 mm to 1 m range).

In standard single mode or multimode optical fibers, the losses atwavelengths less than 1300 nm are dominated by Rayleigh scattering.These Rayleigh scattering loss Ls is determined by the properties of thematerial and are typically about 20 dB/km for visible wavelengths(400-700 nm). Rayleigh scattering losses also have a strong wavelengthdependence (i.e., L_(S) ∝1/λ⁴, see FIG. 4B, comparative fiber A), whichmeans that at least about 1 km to 2 km of the fiber is needed todissipate more than 95% of the input light. Shorter lengths of suchfiber would result in lower illumination efficiency, while using longlengths (1 km to 2 km, or more) can be more costly and can be difficultto manage. The long lengths of fiber, when used in a bioreactor or otherillumination system may be cumbersome to install.

In certain configurations of lighting applications it is desirable touse shorter lengths of fiber, for example, 1-100 meters. This requiresan increase of scattering loss from the fiber, while being able tomaintain good angular scattering properties (uniform dissipation oflight away from the axis of the fiber) and good bending performance toavoid bright spots at fiber bends. A desirable attribute of at leastsome of the embodiments of present invention described herein is uniformand high illumination along the length of the fiber illuminator. Becausethe optical fiber is flexible, it allows a wide variety of theillumination shapes to be deployed. It is preferable to have no brightspots (due to elevated bend losses) at the bending points of the fiber,such that the illumination provided by the fiber does not vary by morethan 30%, preferably less than 20% and more preferably less than 10%.For example, in at least some embodiments, the average scattering lossof the fiber is greater than 50 dB/km, and the scattering loss does notvary more than 30% (i.e., the scattering loss is within ±30% of theaverage scattering loss) over any given fiber segment of 0.2 m length.According to at least some embodiments, the average scattering loss ofthe fiber is greater than 50 dB/km, and the scattering loss does notvary more than 30% over the fiber segments of less than 0.05 m length.According to at least some embodiments, the average scattering loss ofthe fiber is greater than 50 dB/km, and the scattering loss does notvary more than 30% (i.e., ±30%) over the fiber segments 0.01 m length.According to at least some embodiments, the average scattering loss ofthe fiber is greater than 50 dB/km, and the scattering loss does notvary more than 20%(i.e., ±20%) and preferably by not more than 10%(i.e., ±10%) over the fiber segments 0.01 m length.

In at least some embodiments, the intensity variation of the integrated(diffused) light intensity coming through sides of the fiber at theillumination wavelength is less than 30% for target length of the fiber,which can be, for example, 0.02-100 m length. It is noted that theintensity of integrated light intensity through sides of the fiber at aspecified illumination wavelength can be varied by incorporatingfluorescence material in the cladding or coating. The wavelength of thelight scattering by the fluorescent material is different from thewavelength of the light propagating in the fiber.

In some the following exemplary embodiments we describe fiber designswith a nano-structured fiber region (region with nano-sized structures)placed in the core area of the fiber, or very close to core. Some of thefiber embodiments have scattering losses in excess of 50 dB/km (forexample, greater than 100 dB/km, greater than 200 dB/km, greater than500 dB/km, greater than 1000 dB/km, greater than 3000 dB/km, greaterthan 5000 dB/km), the scattering loss (and thus illumination, or lightradiated by these fibers) is uniform in angular space.

In order to reduce or to eliminate bright spots as bends in the fiber,it is desirable that the increase in attenuation at a 90° bend in thefiber is less than 5 dB/turn (for example, less than 3 dB/turn, lessthan 2 dB/turn, less than 1 dB/turn) when the bend diameter is less than50 mm. In exemplary embodiment, these low bend losses are achieved ateven smaller bend diameters, for example, less than 20 mm, less than 10mm, and even less than 5 mm Preferably, the total increase inattenuation is less than 1 dB per 90 degree turn at a bend radius of 5mm.

Preferably, according to some embodiments, the bending loss is equal toor is lower than intrinsic scattering loss from the core of the straightfiber. The intrinsic scattering is predominantly due to scattering fromthe nano-sized structures. Thus, according to at least the bendinsensitive embodiments of optical fiber, the bend loss does not exceedthe intrinsic scattering for the fiber. However, because the scatteringlevel is a function of bending diameter, the bending deployment of thefiber depends on its scattering level. For example, in some of theembodiments, the fiber has a bend loss less than 3 dB/turn, preferablyless than 2 dB/turn, and the fiber can be bent in an arc with a radiusas small as 5 mm radius without forming bright spots.

FIG. 1 is a schematic side view of a section of an example embodiment ofa light diffusing fiber with a plurality of voids in the core of thelight diffusing optical fiber (hereinafter “fiber”) 12 having a centralaxis (“centerline”) 16. FIG. 2 is a schematic cross-section of lightdiffusing optical fiber 12 as viewed along the direction 2-2 in FIG. 1.Light diffusing fiber 12 can be, for example, any one of the varioustypes of optical fiber with a nano-structured fiber region havingperiodic or non-periodic nano-sized structures 32 (for example voids).In an example embodiment, fiber 12 includes a core 20 divided into threesections or regions. These core regions are: a solid central portion 22,a nano-structured ring portion (inner annular core region) 26, andouter, solid portion 28 surrounding the inner annular core region 26. Acladding region 40 (“cladding”) surrounds the annular core 20 and has anouter surface. The cladding 40 may have low refractive index to providea high numerical aperture (NA). The cladding 40 can be, for example, alow index polymer such as UV or thermally curable fluoroacrylate orsilicone.

An optional coating 44 surrounds the cladding 40. Coating 44 may includea low modulus primary coating layer and a high modulus secondary coatinglayer. In at least some embodiments, coating layer 44 comprises apolymer coating such as an acrylate-based or silicone based polymer. Inat least some embodiments, the coating has a constant diameter along thelength of the fiber.

In other exemplary embodiments described below, coating 44 is designedto enhance the distribution and/or the nature of “radiated light” thatpasses from core 20 through cladding 40. The outer surface of thecladding 40 or the of the outer of optional coating 44 represents the“sides” 48 of fiber 12 through which light traveling in the fiber ismade to exit via scattering, as described herein.

A protective cover or sheath (not shown) optionally covers cladding 40.Fiber 12 may include a fluorinated cladding 40, but the fluorinatedcladding is not needed if the fibers are to be used in short-lengthapplications where leakage losses do not degrade the illuminationproperties.

In some exemplary embodiments, the core region 26 of light diffusingfiber 12 comprises a glass matrix (“glass”) 31 with a plurality ofnon-periodically disposed nano-sized structures (e.g., “voids”) 32situated therein, such as the example voids shown in detail in themagnified inset of FIG. 2. In another example embodiment, voids 32 maybe periodically disposed, such as in a photonic crystal optical fiber,wherein the voids typically have diameters between about 1×10⁻⁶ m and1×10⁻⁵ m. Voids 32 may also be non-periodically or randomly disposed. Insome exemplary embodiment, glass 31 in region 26 is fluorine-dopedsilica, while in other embodiment the glass is undoped pure silica.Preferably the diameters of the voids are at least 10 nm.

The nano-sized structures 32 scatter the light away from the core 20 andtoward the outer surface of the fiber. The scattered light is then“diffused” through of the outer surface of the fiber 12 to provide thedesired illumination. That is, most of the light is diffused (viascattering) through the sides of the fiber 12, along the fiber length.Preferably, the fiber emits substantially uniform radiation over itslength, and the fiber has a scattering-induced attenuation of greaterthan 50 dB/km in the wavelength(s) of the emitted radiation(illumination wavelength). Preferably, the scattering-inducedattenuation is greater than 100 dB/km for this wavelength. In someembodiments, the scattering-induced attenuation is greater than 500dB/km for this wavelength, and in some embodiments is 1000 dB/km,greater than 2000 dB/km, and greater than 5000 dB/km. These highscattering losses are about 2.5 to 250 times higher than the Rayleighscattering losses in standard single mode and multimode optical fibers.

Glass in core regions 22 and 28 may include updopants, such as Ge, Al,and/or P. By “non-periodically disposed” or “non-periodic distribution,”it is meant that when one takes a cross-section of the optical fiber(such as shown in FIG. 2), the voids 32 are randomly or non-periodicallydistributed across a portion of the fiber. Similar cross sections takenat different points along the length of the fiber will reveal differentcross-sectional void patterns, i.e., various cross sections will havedifferent voids patterns, wherein the distributions of voids and sizesof voids do not match. That is, the voids are non-periodic, i.e., theyare not periodically disposed within the fiber structure. These voidsare stretched (elongated) along the length (i.e. parallel to thelongitudinal axis) of the optical fiber, but do not extend the entirelength of the entire fiber for typical lengths of transmission fiber.While not wishing to be bound by theory, it is believed that the voidsextend less than 10 meters, and in many cases less than 1 meter alongthe length of the fiber.

The light diffusing fiber 12 as used herein in the illumination systemdiscussed below can be made by methods which utilize preformconsolidation conditions which result in a significant amount of gasesbeing trapped in the consolidated glass blank, thereby causing theformation of voids in the consolidated glass optical fiber preform.Rather than taking steps to remove these voids, the resultant preform isused to form an optical fiber with voids, or nano-sized structures,therein. The resultant fiber's nano-sized structures or voids areutilized to scatter or guide the light out of the fiber, via its sides,along the fiber length. That is, the light is guided away from the core20, through the outer surface of the fiber, to provide the desiredillumination.

As used herein, the diameter of a nano-sized structure such as void isthe longest line segment whose endpoints a) when the optical fiber isviewed in perpendicular cross-section transverse to the longitudinalaxis of the fiber. Method of making optical fibers with nano-sized voidsis described, for example, in U.S. patent application Ser. No.11/583,098, which is incorporated herein by reference.

As described above, in some embodiments of fiber 12, core sections 22and 28 comprise silica doped with germanium, i.e., germania-dopedsilica. Dopants other than germanium, singly or in combination, may beemployed within the core, and particularly at or near the centerline 16,of the optical fiber to obtain the desired refractive index and density.In at least some embodiments, the relative refractive index profile ofthe optical fiber disclosed herein is non-negative in sections 22 and28. These dopants may be, for example, Al, Ti, P, Ge, or a combinationthereof In at least some embodiments, the optical fiber contains noindex-decreasing dopants in the core. In some embodiments, the relativerefractive index profile of the optical fiber disclosed herein isnon-negative in sections 22, 24 and 28.

In some examples of fiber 12 as used herein, the core 20 comprises puresilica. In one embodiment, a preferred attribute of the fiber is theability to scatter light out of the fiber (to diffuse light) in thedesired spectral range to which biological material is sensitive. Inanother embodiment, the scattered light may be used for decorativeaccents and white light applications. The amount of the loss viascattering can be increased by changing the properties of the glass inthe fiber, the width of the nano-structured region 26, and the size andthe density of the nano-sized structures.

In some examples of fiber 12 as used herein, core 20 is a graded-indexcore, and preferably, the refractive index profile of the core has aparabolic (or substantially parabolic) shape; for example, in someembodiments, the refractive index profile of core 20 has an a shape withan a value of about 2, preferably between 1.8 and 2.3 as measured at 850nm. In other embodiments, one or more segments of the refractive indexprofile have a substantially step index shape with an a value greaterthan 8, more preferably greater than 10 even more preferably greaterthan 20 as measured at 850 nm. In some embodiments, the refractive indexof the core may have a centerline dip, wherein the maximum refractiveindex of the core, and the maximum refractive index of the entireoptical fiber, is located a small distance away from centerline 16, butin other embodiments the refractive index of the core has no centerlinedip, and the maximum refractive index of the core, and the maximumrefractive index of the entire optical fiber, is located at thecenterline.

In an exemplary embodiment, fiber 12 has a silica-based core 20 anddepressed index (relative to silica) polymer cladding 40. The low indexpolymer cladding 40 preferably has a relative refractive index that isnegative, more preferably less than −0.5% and even more preferably lessthan −1%. In some exemplary embodiments cladding 40 has thickness of 20μm or more. In some exemplary embodiments cladding 40 has a lowedrefractive index than than the core, and a thickness of 10 μm or more(e.g., 20 μm or more). In some exemplary embodiments, the cladding hasan outer diameter 2 times Rmax, e.g., of about 125 μm (e.g., 120 μm to130 μm, or 123 μm to 128 μm). In other embodiments the cladding has thediameter that is less than 120 for example 60 or 80 μm. In otherembodiments the outer diameter of the cladding is greater than 200 μm,greater than 300 μm, or greater than 500 μm. In some embodiments, theouter diameter of the cladding has a constant diameter along the lengthof fiber 12. In some embodiments, the refractive index of fiber 12 hasradial symmetry. Preferably, the outer diameter 2R3 of core 20 isconstant along the length of the fiber. Preferably the outer diametersof core sections 22, 26, 28 are also constant along the length of thefiber. By constant, we mean that the variations in the diameter withrespect to the mean value are less than 10%, preferably less than 5% andmore preferably less than 2%. FIG. 3A is a plot of the exemplaryrelative refractive index Δ versus fiber radius for an example fiber 12shown in FIG. 2 (solid line). The core 20 may also have a graded coreprofile, with α-profile having, for example, α-value between 1.8 and 2.3(e.g., 1.8 to 2.1).

FIG. 3A is a plot of the exemplary relative refractive index Δ versusfiber radius for an example fiber 12 shown in FIG. 2 (solid line). Thecore 20 may also have a graded core profile, characterized, for example,by an α-value between 1.7 and 2.3 (e.g., 1.8 to 2.3). An alternativeexemplary refractive index profile is illustrated by the dashed lines.Core region 22 extends radially outwardly from the centerline to itsouter radius, R1, and has a relative refractive index profile Δ₁(r)corresponding to a maximum refractive index n1 (and relative refractiveindex percent Δ_(1MAX)). In this embodiment, the reference index n_(REF)is the refractive index at the cladding. The second core region(nano-structured region) 26 has minimum refractive index n2, a relativerefractive index profile Δ2(r), a maximum relative refractive index Δ2_(MAX), and a minimum relative refractive index Δ2 _(MIN), where in someembodiments Δ2 _(MAX)=Δ2 _(MIN). The third core region 28 has a maximumrefractive index n3, a a relative refractive index profile Δ3(r) with amaximum relative refractive index Δ3 _(MAX) and a minimum relativerefractive index Δ3 _(MIN), where in some embodiments Δ3 _(MAX)=Δ3_(MIN). In this embodiment the annular cladding 40 has a refractiveindex n4, a relative refractive index profile Δ4(r) with a maximumrelative refractive index Δ4 _(MAX), and a minimum relative refractiveindex Δ4 _(MIN). In some embodiments Δ4 _(MAX)=Δ4 _(MIN). In someembodiments, Δ1 _(MAX)>Δ4 _(MAX) and Δ3 _(MAX)>Δ4 _(MAX). In someembodiments Δ2 _(MIN)>Δ4 _(MAX) In the embodiment shown in FIGS. 2 and3A, Δ1 _(MAX)>Δ3 _(MAX)>Δ2 _(MAX)>Δ4 _(MAX) In this embodiment therefractive indices of the regions have the following relationshipn1>n3>n2>n4.

In some embodiments, core regions 22, 28 have a substantially constantrefractive index profile, as shown in FIG. 3A with a constant Δ1(r) andΔ3(r). In some of these embodiments, Δ2(r) is either slightly positive(0<Δ2(r)<0.1%), negative (−0.1%<Δ2(r)<0), or 0%. In some embodiments theabsolute magnitude of Δ2(r) is less than 0.1%, preferably less than0.05%. In some embodiments, the outer cladding region 40 has asubstantially constant refractive index profile, as shown in FIG. 3Awith a constant Δ4(r). In some of these embodiments, Δ4(r)=0%. The coresection 22 has a refractive index where Δ1(r)≥0%. In some embodiments,the void-filled region 26 has a relative refractive index profile Δ2(r)having a negative refractive index with absolute magnitude less than0.05%, and Δ3(r) of the core region 28 can be, for example, positive orzero. In at least some embodiments, n1>n2 and n3>n4.

In some embodiments the cladding 40 has a refractive index−0.05%<Δ4(r)<0.05%. In other embodiments, the cladding 40 and the coreportions portion 20, 26, and 28 may comprise pure (undoped) silica.

In some embodiments, the cladding 40 comprises pure or F-doped silica.In some embodiments, the cladding 40 comprises pure low index polymer.In some embodiments, nano-structured region 26 comprises pure silicacomprising a plurality of voids 32. Preferably, the minimum relativerefractive index and the average effective relative refractive index,taking into account the presence of any voids, of nano-structured region26 are both less than −0.1%. The voids or voids 32 may contain one ormore gases, such as argon, nitrogen, oxygen, krypton, or SO₂ or cancontain a vacuum with substantially no gas. However, regardless of thepresence or absence of any gas, the average refractive index innano-structured region 26 is lowered due to the presence of voids 32.Voids 32 can be randomly or non-periodically disposed in thenano-structured region 26, and in other embodiments, the voids aredisposed periodically therein.

In some embodiments, the plurality of voids 32 comprises a plurality ofnon-periodically disposed voids and a plurality of periodically disposedvoids.

In example embodiments, core section 22 comprises germania doped silica,core inner annular region 28 comprises pure silica, and the claddingannular region 40 comprises a glass or a low index polymer. In some ofthese embodiments, nano-structured region 26 comprises a plurality ofvoids 32 in pure silica; and in yet others of these embodiments,nano-structured region 26 comprises a plurality of voids 32 influorine-doped silica.

In some embodiments, the outer radius, Rc, of core is greater than 10 μmand less than 600 μm. In some embodiments, the outer radius Rc of coreis greater than 30 μm and/or less than 400 μm. For example, Rc may be125 μm to 300 μm. In other embodiments, the outer radius Rc of the core20 (please note that in the embodiment shown in FIG. 3A, Rc=R3) islarger than 50 μm and less than 250 μm. The central portion 22 of thecore 20 has a radius in the range 0.1Rc≤R₁<0.9Rc, preferably0.5Rc≤R₁≤09Rc. The width W2 of the nano-structured ring region 26 ispreferably 0.05Rc≤W2≤0.9Rc, preferably 0.1Rc≤W2≤0.9Rc, and in someembodiments 0.5Rc≤W2≤0.9Rc (a wider nano-structured region gives ahigher scattering-induced attenuation, for the same density ofnano-sized structures). The solid glass core region 28 has a width Ws=W3such that 0.1Rc>W3>0.9Rc. Each section of the core 20 comprises silicabased glass. The radial width W₂ of nano-structured region 26 ispreferably greater than 1 μm. For example, W₂ may be 5 μm to 300 μm, andpreferably 200 μm or less. In some embodiments, W₂ is greater than 2 μmand less than 100 μm. In other embodiments, W2 is greater than 2 μm andless than 50 μm. In other embodiments, W₂ is greater than 2 μm and lessthan 20 μm. In some embodiments, W₂ is at least 7 μm. In otherembodiments, W₂ is greater than 2 μm and less than 12 μm. The width W₃of core region 28 is (R3−R2) and its midpoint R_(3MID) is (R2+R3)/2. Insome embodiments, W₃ is greater than 1 um and less than 100 μm.

The numerical aperture (NA) of fiber 12 is preferably equal to orgreater than the NA of a light source directing light into the fiber.Preferably the numerical aperture (NA) of fiber 12 is greater than 0.2,in some embodiments greater than 0.3, and even more preferably greaterthan 0.4.

In some embodiments, the core outer radius R1 of the first core region22 is preferably not less than 24 μm and not more than 50 μm, i.e. thecore diameter is between about 48 and 100 μm. In other embodiments,R1>24 microns; in still other embodiments, R1>30 microns; in yet otherembodiments, R1>40 microns.

In some embodiments, |Δ₂(r)|<0.025% for more than 50% of the radialwidth of the annular inner portion 26, and in other embodiments|Δ₂(r)|<0.01% for more than 50% of the radial width of region 26. Thedepressed-index annular portion 26 begins where the relative refractiveindex of the cladding first reaches a value of less than −0.05%, goingradially outwardly from the centerline. In some embodiments, thecladding 40 has a relative refractive index profile Δ4(r) having amaximum absolute magnitude less than 0.1%, and in this embodiment Δ4_(MAX)<0.05% and Δ4 _(MIN)>−0.05%, and the depressed-index annularportion 26 ends where the outermost void is found.

Cladding structure 40 extends to a radius R4, which is also theoutermost periphery of the optical fiber. In some embodiments, the widthof the cladding, R4-R3, is greater than 20 μm; in other embodimentsR4−R3 is at least 50 μm, and in some embodiments, R4−R3 is at least 70μm.

In another embodiment, the entire core 20 is nano-structured (filledwith voids, for example), and the core 20 is surrounded by the cladding40. The core 20 may have a “step” refractive index delta, or may have agraded core profile, with α-profile having, for example, α-value between1.8 and 2.3.

Preparation of optical preform and fibers for examples shown in FIGS.3C, 4A and 6-8 were as follows: In this exemplary embodiment, 470 gramsof SiO₂ (0.5 g/cc density) soot were deposited via outside vapordeposition (OVD) onto a fully consolidated 1 meter long, 20 mm diameterpure silica void-free core cane, resulting in a preform assembly(sometimes referred to as a preform, or an optical preform) comprising aconsolidated void-free silica core region which was surrounded by a sootsilica region. The soot cladding of this perform assembly was thensintered as follows. The preform assembly was first dried for 2 hours inan atmosphere comprising helium and 3 percent chlorine (all percentgases by volume) at 1100° C. in the upper-zone part of the furnace,followed by down driving at 200 mm/min (corresponding to approximately a100° C./min temperature increase for the outside of the soot preformduring the downdrive process) through a hot zone set at approximately1500° C. in a 100 percent SO₂ (by volume) sintering atmosphere. Thepreform assembly was then down driven again (i.e., a second time)through the hot zone at the rate of 100 mm/min (corresponding to anapproximately 50° C./min temperature increase for the outside of thesoot preform during the downdrive process). The preform assembly wasthen down driven again (i.e., a third time) through the hot zone at therate of 50 mm/min (corresponding to an approximately 25° C./mintemperature increase for the outside of the soot preform during thedowndrive process). The preform assembly was then down driven again(i.e., a fourth time) through the hot zone at the rate of 25 mm/min(corresponding to an approximately 12.5° C./min temperature increase forthe outside of the soot preform during the downdrive process), thenfinally sintered at 6 mm/min (approximately 3° C./min heat up rate) inorder to sinter the soot into an SO₂-seeded silica overclad preform.Following each downdrive step, the preform assembly was updriven at 200mm/min into the upper-zone part of the furnace (which remained set at1100° C.). The first series of higher downfeed rate are employed toglaze the outside of the optical fiber preform, which facilitatestrapping of the gases in the preform. The preform was then placed for 24hours in an argon purged holding oven set at 1000° C. to outgas anyremaining helium in the preform. This preform was then redrawn in anargon atmosphere on a conventional graphite redraw furnace set atapproximately 1700° C. into void-free SiO₂ core, SO₂-seeded (i.e.,containing the non-periodically located voids containing SO₂ gas) silicaoverclad canes which were 10 mm in diameter and 1 meter long.

One of the 10 mm canes was placed back in a lathe where about 190 gramsof additional SiO₂ (0.52 g/cc density) soot was deposited via OVD. Thesoot of this cladding (which may be called overcladding) for thisassembly was then sintered as follows. The assembly was first dried for2 hours in an atmosphere consisting of helium and 3 percent chlorine at1100° C. followed by down driving at 5 mm/min through a hot zone set at1500° C. in a 100% helium (by volume) atmosphere in order to sinter thesoot to a germania containing void-free silica core, silica SO₂-seededring (i.e. silica with voids containing SO₂), and void-free overcladpreform. The preform was placed for 24 hours in an argon purged holdingoven set at 1000° C. to outgas any remaining helium from the preform.The optical fiber preform was drawn to 3 km lengths of 125 microndiameter optical fiber at approximately 1900° C. to 2000° C. in a heliumatmosphere on a graphite resistance furnace. The temperature of theoptical preform was controlled by monitoring and controlling the opticalfiber tension; in this embodiment the fiber tension was held at onevalue between 30 and 600 grams during each portion (e.g., 3 km lengths)of a fiber draw run. The fiber was coated with a low index silicon basedcoating during the draw process.

Another 10 mm void-free silica core SO₂-seeded silica overclad canesdescribed above (i.e., a second cane) was utilized to manufacture theoptical preform and fibers for examples shown in FIG. 4B. Morespecifically, the second 10 mm void-free silica core SO₂-seeded silicaoverclad cane was placed back in a lathe where about 3750 grams ofadditional SiO₂ (0.67 g/cc density) soot are deposited via OVD. The sootof this cladding (which may be called overcladding for this assembly)was then sintered as follows. The assembly was first dried for 2 hoursin an atmosphere comprising of helium and 3 percent chlorine at 1100°C., followed by down driving at 5 mm/min through a hot zone set at 1500°C. in a 100% helium (by volume) atmosphere in order to sinter the sootso as to produce preform comprising germania containing void-free silicacore, silica SO₂-seeded ring (i.e. silica with voids containing SO₂),and void-free overclad. The resultant optical fiber preform was placedfor 24 hours in an argon purged holding oven set at 1000° C. to outgasany remaining helium from the preform. Finally, the optical fiberpreform was drawn to 5 km lengths of 125 micron diameter optical fiberand coated with the low index polymer as described above.

FIG. 3B illustrates schematically yet another exemplary embodiment oflight diffusing fiber 12. The fiber of FIG. 3B includes a core 20 with arelative refractive index Δ₁, a nano-structured region 26′ situated overand surrounding the core 20. The core 20 may have a “step” indexprofile, or a graded core profile, with α-profile having, for example,α-value between 1.8 and 2.3.

In this exemplary embodiment (see FIG. 3B) the nano-structured region26′ is an annular ring with a plurality of voids 32. In this embodiment,the width of region 26′ can be as small as 1-2 um, and may have anegative average relative refractive index Δ₂. Cladding 40 surrounds thenano-structured region 26′. The (radial) width of cladding 40 may be assmall as 1 μm, and the cladding may have either a negative, a positiveor 0% relative refractive index, (relative to pure silica). The maindifference between examples in FIGS. 3A and 3B is that nano-structuredregion in shown in FIG. 3A is located in the core 20 of the lightdiffusing fiber 12, and in FIG. 3B it is located at the core/cladinterface. The depressed-index annular portion 26′ begins where therelative refractive index of the core first reaches a value of less than−0.05%, going radially outwardly from the centerline. In the embodimentof FIG. 3B, the cladding 40 has a relative refractive index profileΔ3(r) having a maximum absolute magnitude less than 0.1%, and in thisembodiment Δ3 _(MAX)<0.05% and Δ3 _(MIN)>−0.05%, and the depressed-indexannular portion 26 ends where the outmost void occurs in the void-filledregion.

In the embodiment shown in FIG. 3B the index of refraction of the core20 is greater than the index of refraction n2 of the annular region 26′,and the index of refraction n1 of the cladding 40 is also greater thanthe index of refraction n2.

FIG. 3C illustrates a core 20 of one embodiment of an optical fiber 12that has been made. This fiber has a first core region 22 with an outerradius R1 of about 33.4 μm, a nano-structured region 26 with an outerradius R2=42.8 μm, a third core region 28 with an outer radius R3=62.5μm, and a polymer cladding 40 with an outer radius R4 (not shown) of82.5 μm). In this embodiment, the material of the core is pure silica(undoped silica), the material for cladding was low index polymer (e.g.,UV curable silicone having a refractive index of 1.413 available fromDow-Corning of Midland, Mich. under product name Q3-6696) which, inconjunction with the glass core, resulted in fiber NA of 0.3. Theoptical fiber 12 had a relatively flat (weak) dependence on wavelength,compared to standard single-mode transmission fiber, such as for exampleSMF-28e^(R) fiber, FIG. 4B. In standard single mode (such as SMF-28^(R))or multimode optical fibers, the losses at wavelengths less than 1300 nmare dominated by Rayleigh scattering. These Rayleigh scattering lossesare determined by the properties of the material and are typically about20 dB/km for visible wavelengths (400-700 nm). The wavelength dependenceof Rayleigh scattering losses is proportional to λ^(−p) with p≈4. Theexponent of the wavelength dependent scattering losses in the fibercomprising at least one nanostructured region is less than 2, preferablyless than 1 over at least 80% (for example greater than 90%) in the 400nm-1100 nm wavelength range. The average spectral attenuation from 400nm to 1100 nm was about 0.4 dB/m when the fiber was drawn with at 40 gtension and was about 0.1 dB/m when the fiber 12 was drawn at 90 gtension. In this embodiment, the nano-sized structures contain SO₂ gas.Applicants found that filled SO₂ voids in the nano-structured ringgreatly contribute to scattering. Furthermore, when SO₂ gas was used toform the nano-structures, it has been discovered that this gas allows athermally reversible loss to be obtained, i.e., below 600° C. thenano-structured fiber scatters light, but above 600° C. the same fiberwill guide light. This unique behavior that SO₂ imparts is alsoreversible, in that upon cooling the same fiber below 600° C., the fiber12 will act as light diffusing fiber and will again generate anobservable scattering effect.

In preferred embodiments, the uniformity of illumination along the fiberlength is controlled such that the minimum scattering illuminationintensity is not less than 0.7 of the maximum scattering illuminationintensity, by controlling fiber tension during the draw process; or byselecting the appropriate draw tension (e.g., between 30 g and 100 g, orbetween 40 g and 90 g)

Accordingly, according to some embodiments, a method of making a lightdiffusing fiber to control uniformity of illumination along the fiberlength wherein the minimum scattering illumination intensity is not lessthan 0.7 the maximum scattering illumination intensity includes the stepof controlling fiber tension during draw process.

The presence of the nano-sized structures in the light diffusing fiber12 creates losses due to optical scattering, and the light scatteringthrough the outer surface of the fiber can be used for illuminationpurposes. FIG. 4A is a plot of the attenuation (loss) in dB/m versuswavelength (nm) for the fiber of FIG. 3C (fiber with SO₂ gas filledvoids). FIG. 4A illustrates that (i) light diffusing fibers 12 canachieve very large scattering losses (and thus can provide highillumination intensity) in the visible wavelength range. The scatteringlosses of the optical fiber 12 also have weak wavelength dependence(L_(s) is proportional to 1/λ^(−p), where p is less than 2, preferablyless than 1, and even more preferably less than 0.5), as compared toregular 125 μm graded index core multi mode comparative fiber A (fiber Ais a step index multimode fiber without the nano-structured region)which has Rayleigh scattering losses of about 0.02 dB/m in the visiblewavelength range, or about 20 dB/km at the wavelength of 500 nm andrelatively strong wavelength dependence of 1/λ⁴). The effect of thetension for the fibers 12 is also illustrated in FIGS. 4A-4B. Morespecifically FIGS. 4A-4B illustrate that the higher fiber draw tensionresults in lower scattering losses, and that lower fiber draw tensionresults in a fiber section with higher scattering loss, i.e., strongerillumination). FIG. 4A depicts attenuation as function of wavelength forlight diffusing fiber 12 (with voids in the core) drawn at differentfiber tensions of 90 and 400 g. FIG. 4B depicts attenuation as functionof wavelength for different light diffusing fiber 12 (with voids in thecore) drawn at different fiber tension, 90 and 40 g, a comparative multimode fiber (fiber A) with normalized loss, and a theoretical fiber with1/λ, loss dependence. (Note, FIG. 4B graph describes wavelengthdependence of the loss. In this example, in order to compare the slopeof the scattering for the light fiber 12 and fiber A, the loss of lowloss fiber (fiber A) was multiplied by a factor of 20, so that the twoplots can be easily shown on the same figure.) Without being bound toany particular theory, it is believed that the increase in thescattering losses when the draw tension decreases, for example from 90 gto 40 g, is due to an increase in the average diameter of thenanostructures. Therefore, this effect of fiber tension could be used toproduce constant attenuation (illumination intensity) along the lengthof the fiber by varying the fiber tension during the draw process. Forexample, a first fiber segment drawn at high tension, T1, with a loss ofα₁dB/m and length, L1, will attenuate the optical power from an inputlevel P0 to P0 exp(−α₁*L1/4.343). A second fiber segment opticallycoupled to the first fiber segment and drawn at lower tension T2 with aloss of α₂ dB/m and length L2 will further attenuate the optical powerfrom P0 exp(−α₁*L1/4.343) to P0 exp(−α₁*L1/4.343) exp(−α₂*L2/4.343). Thelengths and attenuations of the first and second fiber segments can beadjusted to provide uniform intensity along the length of theconcatenated fiber.

One of the advantages of light diffusing fibers 12 is their ability toprovide uniform illumination along the length of the light diffusingfiber. FIG. 5 illustrates the arrangement of fiber 12 that results inuniform illumination along the length of the fiber and utilizes twolight passes in the single light diffusing fiber 12. In this arrangementa mirror M is placed at the end of the light diffusing fiber 12. Theinput light provided by the light source 150 to the light diffusingfiber 12 propagates along the axis of the light diffusing fiber 12, andthe remaining light is reflected by the mirror and propagates back alongthe axis of the fiber 12 towards the input. If the attenuation andlength of the fiber 12 are chosen properly, the light output powerprovided back to the light source is less than a 2%-5% percent of theoriginal light power. The scattering loss intensity for fiber withconstant loss distribution (see FIG. 4A) may be higher in the beginningof the fiber and weaker at the end of the fiber. However, if the lightdiffusing fiber 12 is drawn with a periodically controlled tension (thetension value is related to the furnace temperature, which may vary from1800° C. to 2100° C.) such that the scattering losses are lower at thebeginning of the fiber, where the intensity is high, and higher at theend, where the intensity is lower, the resulting scattering intensitycan be made less variable, or constant (for example, as shown in FIG.6A, example C). The fiber draw tension may be controlled and varied, forexample, between 40 g and 400 g, thus providing a wide range ofscattering-induced attenuation (e.g., up to 6 times). The mirror M inFIG. 5 may also be replaced by a second light source with power densityoutput that similar to that of the first light source (within a factorof 2, i.e., in the range of 50% to 200%) to not only create a moreuniform illumination, but also to increase the quantity of lightscattered by the fiber.

One aspect of an exemplary embodiment of the bioreactor/illuminationsystem is that the angular distribution of the scattering lightintensity is uniform or nearly uniform in angular space. The lightscattering axially from the surface of the fiber has a variationrelative to the mean scattering intensity that is less than 50%,preferably less than 30%, preferably less than 20% and more preferablyless than 10%. The dominant scattering mechanism in conventionalsilica-based optical fibers without nano-sized structures is Rayleighscattering, which has a broad angular distribution. Fibers 12 in whichthere are additional scattering losses due to voids in nano-structuredring may have a strong forward component, as shown in FIG. 6A(embodiments a and b) and FIG. 6B (embodiment a′). This distribution,however, can be corrected by placing a scattering material on the top ofcoating of the light diffusing fiber 12. Light diffusing fibers madewith coating containing TiO₂ based white ink (see FIG. 6B, embodimentb′) provide an angular distribution of scattered light that issignificantly less forward biased. With an additional thicker layer ofTiO₂ ink (e.g., 1-5 μm) it is possible to further reduce the forwardscattering component, thereby increasing the uniformity of the angularintensity distribution. However, as shown in FIG. 7, if the illuminatordesign may utilize fiber(s) optically coupled to a back reflectivemirror or additional light source (see FIG. 5), so even if the fiber hasno TiO₂ based white ink coating layer this configuration providesrelatively flat (i.e., very uniform) angular scattering intensity (seeFIG. 6A). In some embodiments, a controlled variation of the ink coating(either thickness of the ink coating or variation of ink concentrationin the coating) along the length of the fiber will provide an additionalway of making more uniform variation in the intensity of the lightscattered form the fiber at large angles (more than 15 degrees).

In some embodiments the ink can be a fluorescent material that convertsscattered light to a longer wavelength of light. In some embodimentswhite light can be emitted (diffused out of the outer surface) by thefiber 12 by coupling the light diffusing fiber 12 with such a coating toa UV light source, for example a 405 nm or 445 nm diode laser. Theangular distribution of fluorescence white light in the exemplaryembodiments is substantially uniform (e.g., 25% to 400%, preferably 50%to 200%, even more preferably 50% to 150%, or 70% to 130%, or 80% to120% in angular space).

Lighting System Configuration

Efficient coupling to low cost light sources such as light emittingdiodes (LEDs) or sunlight requires the fiber to have a high NA and largecore diameter. With a design similar to that shown in FIG. 2 the size ofthe multimode core 20 can be maximized, and may have a radius up to 500μm. The cladding thickness may be much smaller, for example, about 15-30μm (e.g., about 20 μm). For example, according to one embodiment, aplurality of light diffusing fibers 12 may be wound around a supportstructure, and each light diffusing optical fiber may be opticallycoupled to either the light source or a plurality of light sources. Theplurality of light diffusing optical fibers 12 can be bundled togetherin at least one of: a ribbon, ribbon stack, or a round bundle. The fiberbundles or ribbons (i.e., collections of multiple fibers) can also bearranged in the shape of the light source in order to increase couplingefficiency. A typical bundle/ribbon structure can include, for example,2-36 light diffusing fibers 12, or may include up to several hundredfibers 12. Cable designs which are assemblies of multiple fibers arewell known and could include ribbons, collections of multiple ribbons,or fibers gathered into a tube. Such fibers may include one or morelight diffusing fibers 12.

Single Fibers

A bright continuous light source coupled into a light diffusing fibercan be utilized for different application such as signs, or displayillumination. If the illumination system utilizes a single fiber 12 withcore diameter of 125-300 μm, a multimode laser diode could be used as alight source for providing light into the fiber 12. An exemplarylighting fixture (bright perimeter illuminator for the display screen)using a single fiber 12 with a reflective coating directing light in onedirection is shown in FIG. 8A. According to some embodiments, single ormultiple fiber illumination with the light diffusing fiber(s) 12 can beutilized in aqueous environments, for example for lighting boat docks,fishing lines or lures, and related applications where the smallflexible size of the light diffusing fiber 12 and the ability to besafely immersed in water are highly desirable. The light diffusing fiber12 may also be useful for exit lighting, illuminating pathways, emittingIR radiation for room detectors, or used a thread in clothing,particularly protective/reflective clothing to further enhancevisibility of the wearer. Examples of the use of the light diffusingfiber 12 in decorative illumination are manifold, but a few examples areuse in appliance lighting and edge effects, automotive/aircraftillumination, or household and furniture illumination.

FIG. 8B illustrates an example embodiment of a biological growth system98 and an illumination system 100 as used in the biological growthsystem, wherein biological chamber 170 is in the form of a flask with aninterior 172. The light source 150 and optical coupling system 160 areconfigured to couple light from the light source into the input ends oflight conducting fiber. The output end of the low-scatter lightconducting fiber 12A is coupled to the input end of the light diffusingfiber 12 (light source fiber). In the embodiment of FIG. 8B, thelight-source fiber 12 is formed from a single counter-wound fiber. It isnoted that the fiber 12 can wound around a support structure to form alight-source fiber portion where guided light is scattered from thefiber outer surface to form an extended light source that emitssubstantially uniform radiation. The bends in the light-source fiberportion are formed to enhance the amount of scattering in the lightdiffusing fiber 12. Counter-winding at least one fiber can increase theuniformity of the radiation by compensating for the effects ofdecreasing emitted radiation along the length of the light-source fiberportion. Multiple fibers 12 can be wound in sequence around a supportstructure, with each fiber coupled to the light source, can be used toform a lengthy extended source. The light diffusing fiber 12 can beconfigured to suit a variety of biological chamber geometries andprovides light to the biological material growth of biological material180. The biological material 180 may be, for example, algae (e.g., algaecolonies, algae blooms) or bacteria (e.g., cyanobacteria). In an exampleembodiment, biological material 180 may be suspended in a support medium184 such as water.

Coatings

In an example embodiment, fiber 12 may include a coating 44 as discussedabove in connection with FIG. 2. In one exemplary embodiment, coating 44includes a hydrophilic coating layer such as a UV-cured acrylate coatingthat provides improved wet adhesion. The coating layer may be UV curablecoatings comprising a low modulus primary coating layer (typically <3MPa) adjacent to the glass and a higher modulus secondary coating layer(typically >50 MPa). The higher modulus secondary coating layer isadjacent to, and situated over the primary (lower modulus) coatinglayer. Other, or additional coatings, applied either as a single layercoating or as a layer in a multi-layer coating may also be utilized.Examples of such materials are hydrophilic coating 44A (not shown) whichserves as a cell growth medium or a coating containing a material toprovide additional scattering to the escaped light. These coatings mayalso serve as a protective covering for the fiber 12.

Exemplary hydrophilic coatings 44A for use in coating 44 are thosecommonly used for improving cell adhesion and growth to surfaces andcontain carboxylic acid functionality and amine functionality (e.g.formulations containing acrylic acid or acrylamides). In addition,hydrophilic coatings 44A may be enhanced by serving as a reservoir fornutrients essential for the growth of biological material.

In some exemplary embodiments, coating 44 includes fluorescent orultraviolet absorbing molecules that serve to modify radiated light.Suitable up or down converter molecules may also be included in thecoating to produce light of differing wavelengths from that of the inputlight source. Ink coating layers may also be applied to alter the coloror hue of the emitted light. Other coating embodiments include moleculescapable of providing additional scattering to the light emitted from thefiber. A further embodiment may be the inclusion of photo-activecatalysts onto the coating that may be used to increase the rate ofphoto-reactions. One example of just such a catalyst is rutile TiO₂, asa photo-catalyst.

According to some embodiments, light diffusing fibers 12 may be enclosedwithin a polymeric, metal, or glass covering (or coatings), wherein saidthe coating or covering has a minimum outer dimension (e.g., diameter)greater than 250 μm. If the fiber(s) has a metal coating, the metalcoating may contain open sections, to allow light to be preferentiallydirected into a given area. These additional coatings or coverings mayalso contain additional compounds to vary the emitted light or catalyzereactions in the same manner as described above for the coatings coatedon the fiber.

As stated above, the light diffusing fiber 12 may comprise a hydrophiliccoating disposed on the outer surface of the optical fiber. Also,fluorescent species (e.g., ultraviolet-absorbing material) may bedisposed in the optical fiber coating, as well as molecules capable ofproviding additional scattering of the emitted light. According to someembodiments the light source coupled to the light diffusing fiber 12generates light in 200 nm to 500 nm wavelength range and the fluorescentmaterial (fluorescent species) in the fiber coating generates eitherwhite, green, red, or NIR (near infrared) light.

Furthermore, an additional coating layer may be provided on the fiberouter surface. This layer may be configured to modify the radiatedlight, alter the interaction of the coating materials. Examples of justsuch a coating would be coatings containing materials such as, but notlimited to, poly (2-acrylamido-2-methanesulfonic acid),ortho-nitrobenzyl groups, or azobenzene moities respectively.

Exemplary Illumination System Configurations

Some exemplary embodiments of an illumination system include: (i) alight source that generates light having at least one wavelength 2,within the 200 nm to 2000 nm range; and (ii) at least one lightdiffusing optical fiber 12. The fiber 12 comprises having a core,cladding, and a plurality of nano-sized structures 32 situated withinthe core or at a core-cladding boundary. This optical fiber furtherincludes an outer surface, and at least one end optically coupled to thelight source. As described above, the light diffusing optical fiber 12is configured to scatter guided light via the nano-sized structures suchas voids away from the core and through the outer surface, to form alight-source fiber portion having a length that emits substantiallyuniform radiation over its length. The light diffusing optical fiber 12has a scattering-induced attenuation greater than 50 dB/km for one ormore wavelength(s) within the 200 nm to 2000 nm range (e.g. 400-700 nm,or 1 μm to 2 μm). The fiber 12 may have a plurality of bends formedtherein so as to preferentially scatter light via the nano-sizedstructures 32 away from the core 20 and through the outer surface withinspecified area(s). Preferably, the deviation of the illuminationintensity of scattered light is less than 30% of the maximum scatteringillumination intensity along the length. According to some embodimentsthe scattering-induced attenuation is between 100 dB/km and 6000 dB/km,or higher. In some embodiments, attenuation due to scattering of fiber12 is 6000 dB/km to 20000 dB/km for the one or more wavelength(s)situated within 200 nm to 2000 nm range. According to some embodimentsfiber 12 has a length between 0.5 m and 100 m and the scattering-inducedattenuation is between 300 dB/km and 5000 dB/km for the one or morewavelength(s) situated within 200 nm to 2000 nm range, and/or is greaterthan 3 dB/length of fiber. In other embodiments, the fiber 12 has alength between 0.1 m and 0.5 m and the scattering-induced attenuation isbetween 5000 dB/km and 20,000 dB/km for the one or more wavelength(s)situated within 200 nm to 2000 nm range. Preferably, the nano-sizedstructures 32 are gas filled voids (e.g., SO₂ filled voids) withdiameter of greater than 10 nm, preferably greater than 50 nm, morepreferably greater than 100 nm. Preferably the fiber cladding is eitherglass, or polymer, and is at least 20 μm thick. The cladding, incombination with said core, provides a NA of 0.2 or greater. Asdescribed above, uniformity of illumination along the fiber length (withabout 30% from maximum intensity, and preferably within about 20% frommaximum intensity, and more preferably within about 10% from maximumintensity) can be accomplished by controlling the fiber tension duringthe draw process. As previously discussed, the uniformity of theillumination can be further reduced by utilizing a reflector coupled tothe end of the fiber that is opposite to the end of the fiber coupled tothe optical source.

Thus, according to some embodiments, the light diffusing fiber 12includes a core at least partially filled with nanostructures forscattering light, a cladding surrounding the core, and ay least onecoating surrounding the cladding. For example, the core and cladding maybe surrounded by a primary and secondary coating layers, and/or by anink layer. In some embodiments the ink layer contains pigments toprovide additional absorption and modify the spectrum of the lightscattered by the fiber (e.g., to provide additional color(s) to thediffused light). In other embodiments, one or more of the coating layerscomprises molecules which convert the wavelength of the lightpropagating through the fiber core such that the light emanating fromthe fiber coating (light diffused by the fiber) is at a differentwavelength. In some embodiments the ink layer and/or the coating layermay comprise phosphor in order to convert the scattered light from thecore into light of differing wavelength(s). In some embodiments thephosphor and/or pigments are dispersed in the primary coating. In someembodiments the pigments are dispersed in the secondary coating, in someembodiments the pigments are dispersed in the primary and secondarycoatings. In some embodiments the phosphor and/or pigments are dispersedin the polymeric cladding. Preferably, the nanostructures are voidsfilled SO₂.

According to some embodiments the optical fiber 12 includes a primarycoating, an optional secondary coating surrounding the primary coatingand/or an ink layer (for example located directly on the cladding, or onone of the coatings. The primary and/or the secondary coating maycomprise at leas one of: pigment, phosphors, fluorescent material, UVabsorbing material, hydrophilic material, light modifying material, or acombination thereof

The plurality of light diffusing fibers 12 can be bundled together in atleast one of a ribbon, ribbon stack, or a round bundle. The fiberbundles or ribbons (i.e., collections of multiple fibers) can also bearranged in the shape of the light source in order to increase couplingefficiency. A typical bundle/ribbon structure can include, for example 2to 36 light diffusing fibers 12, or, with overstacking of fibers, mayinclude up to several hundreds of fibers 12.

As stated above, the optical fiber may comprise a hydrophilic coatingdisposed on the outer surface of the optical fiber. Alternatively, ahydrophilic coating may be disposed on the outer surface of the fiberribbon. A ribbon may also be arranged in the shape of the light source,to provide better coupling between the light diffusing fibers 12 and thelight source. An advantage derived from the ribbon structure is thatwinding of the individual fibers may not be necessary, because theribbons may form bent structures such as waves, helices, or spiralsthereby allowing light to scatter into desired areas. Furthermore, theuse of multi-fiber ribbons affords the possibility of having largestacks of ribbons. Such ribbon stacks would provide a more concentratedamount of light, and also open the possibility to the use of differentlight sources, such as red lasers, sunlight, light emitting diodes, orguidance of point light sources. For example, according to oneembodiment, a plurality of light diffusing optical fibers 12 may beoptically coupled to either a single light source or a plurality oflight sources, while the light diffusing optical fibers 12 are bundledtogether in at least one of a ribbon, ribbon stack, or a round bundle.Furthermore the bundles or ribbons of light diffusing fibers 12 may beconnected to a light source(s) by a transmission fiber capable ofdirecting the light towards the light diffusing fiber with a minimum ofloss. This latter configuration can be expected to be very useful forremote lighting applications where light is gathered from a sourcedistant from the area where light is to be delivered.

According to some embodiments, a light diffusing optical fiber includes:

-   -   1) a core, a cladding, and a plurality of nano-sized structures        situated within said core or at a core-cladding boundary, the        optical fiber further including an outer surface and is        configured to (i) scatter guided light via said nano-sized        structures away from the core and through the outer        surface, (ii) have a scattering-induced attenuation greater than        50 dB/km at illumination wavelength; and 2) one or more        coatings, such that either the cladding or at least one coating        includes phosphor or pigments. According to some embodiments        these pigments may be capable of altering the wavelength of the        light such that the illumination (diffused light) provided by        the outer surface of the fiber is of a different wavelength from        that of the light propagating through fiber core. Preferably,        the nanostructures are voids filled SO₂.

According to some embodiments, a light diffusing optical fiber includes:a core, a cladding, and a plurality of nano-sized structures situatedwithin said core or at a core-cladding boundary. The optical fiberfurther includes an outer surface and is configured to (i) scatterguided light via said nano-sized structures away from the core andthrough the outer surface, (ii) have a scattering-induced attenuationgreater than 50 dB/km at illumination wavelength; wherein the entirecore includes nano-sized structures. Such fiber may optionally includeat least one coating, such that either the cladding or at least onecoating includes phosphor or pigments. According to some embodiments thenanostructures are voids filled SO₂.

According to some embodiments, a light diffusing optical fiber includes:

a core, and a plurality of nano-sized structures situated within saidcore such that the entire core includes nano-structures, the opticalfiber further including an outer surface and is configured to (i)scatter guided light via said nano-sized structures away from the coreand through the outer surface, (ii) have a scattering-inducedattenuation greater than 50 dB/km at illumination wavelength, whereinthe fiber does not include cladding. According to some embodiments thenanostructures are voids filled SO₂. The SO₂ filled voids in thenano-structured area greatly contribute to scattering (improvescattering).

According to some embodiments, a light diffusing optical fiber includes:

a core, and a plurality of nano-sized structures situated within saidcore such that the entire core includes nano-structures, said opticalfiber further including an outer surface and is configured to (i)scatter guided light via said nano-sized structures away from the coreand through the outer surface, (ii) have a scattering-inducedattenuation greater than 50 dB/km at illumination wavelength whereinsaid fiber does not include cladding. According to some embodiments thefiber includes at least one coating such that either the cladding or thecoating includes phosphor or pigments. According to some embodiments thenanostructures are voids filled SO₂. As stated above, the SO₂ filledvoids in the nano-structured area greatly contribute to scattering(improve scattering).

It is to be understood that the foregoing description is exemplary ofthe invention only and is intended to provide an overview for theunderstanding of the nature and character of the invention as it isdefined by the claims. The accompanying drawings are included to providea further understanding of the invention and are incorporated andconstitute part of this specification. The drawings illustrate variousfeatures and embodiments of the invention which, together with theirdescription, serve to explain the principals and operation of theinvention. It will become apparent to those skilled in the art thatvarious modifications to the preferred embodiment of the invention asdescribed herein can be made without departing from the spirit or scopeof the invention as defined by the appended claims.

What is claimed is:
 1. A light diffusing optical fiber having an outersurface, said fiber comprising: (i) a glass core, comprising (a) aregion with a plurality of nano-sized structures within said coreconfigured to scatter guided light via said nano-sized structurestowards the outer surface, such that said light diffusing optical fiberhas a scattering-induced attenuation greater than 50 dB/km atillumination wavelength; and (b) a solid glass region surrounding saidregion with a plurality of nano-sized structures having a higherrefractive index delta than the region with a plurality of nano-sizedstructures; and (ii) a low index cladding surrounding said core, saidcladding having a lower refractive index delta than said solid glassregion.
 2. The light diffusing optical fiber according to claim 1,wherein said low index cladding has a refractive index delta of lessthan −0.5% relative to silica.
 3. The light diffusing optical fiberaccording to claim 1, wherein said low index cladding is glass.
 4. Thelight diffusing optical fiber according to claim 1, wherein said lowindex cladding is a polymer.
 5. The light diffusing optical fiberaccording to claim 4, wherein said low index cladding is: (i) siliconebased; or (ii) a fluoroacrylate.
 6. The light diffusing optical fiberaccording to claim 4, wherein said low index cladding is a fluorinatedcladding.
 7. The light diffusing optical fiber according to claim 1,further including phosphorus containing coating surrounding saidcladding.
 8. The light diffusing optical fiber of claim 1, said fibercore further having: (i) a core diameter greater than 50 μm and lessthan 500 μm; and/or (ii) numerical aperture NA, wherein NA>0.2.
 9. Thelight diffusing optical fiber of claim 1, wherein said nano-sizedstructures are gas filled voids with diameter of greater than 10 nm. 10.A cable assembly of multiple fibers of claim 9 further comprisingcollections of multiple ribbons, or fibers gathered into a tube.
 11. Thelight diffusing optical fiber of claim 1, said plurality of nano-sizedstructures is the nano-structured region that has a width of at least 7μm; and wherein said cladding diameter is at least 125 μm.
 12. The lightdiffusing optical fiber of claim 1, wherein said core has an outerdiameter Rc, said core includes: (i) a solid inner core section with anouter diameter R₁, such that 0.05Rc>R₁>0.9Rc; (ii) nano-structuredregion having a width W₂ wherein 0.05Rc>W₂>0.9Rc; and (iii) outer solidcore region having a width between 0.1Rc>Ws>0.9Rc, wherein each sectionof said core comprises silica glass.
 13. The light diffusing opticalfiber of claim 12, said cladding comprises a low index polymer claddingsurrounding core; and a polymer coating surrounding said cladding, saidpolymer coating.
 14. The light diffusing optical fiber of claim 1,wherein said fiber is configured to be capable of guiding said lightsuch that radiation out of said outer surface is substantially uniform,such that the difference between the minimum and maximum scatteringillumination intensity is less than 30% of the maximum scatteringillumination intensity.
 15. The light diffusing optical fiber of claim1, wherein said fiber having scattering-induced attenuation is 100 dB/kmto 6000 dB/km at illumination wavelength.
 16. The fiber bundles orribbons comprising multiple light diffusing fibers according to claim 1.17. The fiber bundles or ribbons of claim 16 containing 2-36 lightdiffusing fibers.
 18. The light diffusing optical fiber comprising: acore, a cladding, and a plurality of nano-sized structures situatedwithin said core or at a core-cladding boundary, said optical fiberfurther including an outer surface and is configured to (i) scatterguided light via said nano-sized structures away from the core andthrough the outer surface, (ii) have a scattering-induced attenuationgreater than 50 dB/km at illumination wavelength; and said fiberincluding at least one coating such that either said cladding or said atleast one coating includes phosphor or pigments, wherein said nano-sizedstructures are situated within one region of said core, and said corefurther comprises a second region without the microstructures adjacentand surrounding the region that comprises nano-sized structures; saidfiber further comprising a low index polymer cladding surrounding saidsecond region; and said at least one coating is a polymer coatingsurrounding said cladding, said polymer coating having a higherrefractive index than said a low index polymer cladding.
 19. The lightdiffusing optical fiber of claim 18, wherein said coating includesphosphorus.
 20. A light diffusing optical fiber having an outer surface,said fiber comprising: (i) a glass core, comprising (a) a region with aplurality of nano-sized structures configured to scatter guided lightvia said nano-sized structures away from the core and towards the outersurface, such that said light diffusing optical fiber has ascattering-induced attenuation greater than 200 dB/km at illuminationwavelength; and (b) a solid glass region surrounding said region with aplurality of nano-sized structures having a higher refractive indexdelta than the region with a plurality of nano-sized structures; and(ii) a low index cladding surrounding said core, said cladding having alower refractive index delta than said solid glass region and isnegative relative to silica; (iii) a coating surrounding said cladding;wherein said coating or cladding includes phosphorus.
 21. The lightdiffusing optical fiber of claim 20, wherein the scattering loss isuniform in angular space.
 22. The light diffusing optical fiber of claim21, wherein said low index polymer cladding has a relative refractiveindex delta with respect to silica that is less than −0.5%.
 23. Thelight diffusing optical fiber of claim 21, wherein said fiber providesillumination; and uniformity of illumination along the fiber length iswith 30% from maximum intensity.
 24. The light diffusing optical fiberof claim 21 wherein said nanostructures are voids filled with gas. 25.The light diffusing optical fiber of claim 20, wherein said fiber has ascattering-induced attenuation not less than 500 dB/km at theillumination wavelength.
 26. The light diffusing optical fiber of claim20, wherein said fiber has a scattering-induced attenuation greater than1000 dB/km at illumination wavelength.
 27. The light diffusing opticalfiber of claim 20, wherein said fiber has a scattering-inducedattenuation greater than 2000 dB/km at illumination wavelength.
 28. Thelight diffusing optical fiber of claim 20, wherein said fiber core issilica or silica doped with Al, Ti, P, Ge, or a combination thereof. 29.The light diffusing optical fiber of claim 20, wherein said fiber coreradius is less than 500 μm.
 30. The light diffusing optical fiber ofclaim 20, wherein said fiber core has a core radius of 125-300 μm. 31.The light diffusing optical fiber of claim 20, wherein the fiber has abend loss less than 3 dB/turn when fiber can be bent in an arc with a 5mm radius.
 32. The light diffusing optical fiber of claim 20 whereinsaid nanostructures are voids filled with SO₂.
 33. A light diffusingoptical fiber, extending along a length, comprising: a core, a low indexcladding surrounding the core, and a plurality of nano- sized structureslocated adjacent to a core-cladding boundary, wherein said core has atleast one region without said nano-sized microstructures adjacent tosaid core-cladding boundary, said light diffusing optical fiber having:(a) an outer surface, wherein the light diffusing optical fiber isconfigured to scatter guided light, via said plurality of nano-sizedstructures, away from the core and through the outer surface, whereinsaid fiber is configured to be capable of guiding said light such thatradiation out of said outer surface is substantially uniform over thelength of the fiber such that there is a difference between minimum andmaximum scattering illumination intensity that is less than 50% of themaximum scattering illumination intensity; (b) a scattering-inducedattenuation, over its length, of greater than 50 dB/km at anillumination wavelength; and (c) at least one polymer coatingsurrounding said low index cladding, said polymer coating having ahigher refractive index than said low index cladding, wherein eithersaid low index cladding or said at least one coating includes aphosphor.
 34. The light diffusing optical fiber of claim 33, whereinsaid low index cladding is a low index polymer.
 35. The light diffusingoptical fiber of claim 34, wherein said low index cladding is siliconebased or a fluoroacrylate.
 36. The light diffusing optical fiber ofclaim 33, wherein the core includes at least one dopant.
 37. The lightdiffusing optical fiber of claim 36, wherein the at least one dopantcomprises aluminum, titanium, phosphorus, germanium, or a combinationthereof.
 38. The light diffusing optical fiber of claim 33, wherein theat least one polymer coating includes a light scattering agent.
 39. Thelight diffusing optical fiber of claim 33, wherein the low indexcladding has a diameter of greater than 60 μm.
 40. The light diffusingoptical fiber of claim 33, wherein the low index cladding has athickness of greater than 10 μm.
 41. The light diffusing optical fiberof claim 33, wherein said nano- sized structures have a cross-sectionalsize of between 10 nm and 1 μm, and a length between 1 mm and 50 m. 42.The light diffusing optical fiber of claim 33, wherein the plurality ofnano-sized structures are a plurality of nano-sized gas-filled voids.43. The light diffusing optical fiber of claim 33, wherein the lightdiffusing optical fiber has a length of between 1 m and and 100 m. 44.The light diffusing optical fiber of claim 33, wherein said fiber isconfigured to be capable of guiding said light such that radiation outof said outer surface is substantially uniform over the length of thefiber such that there is a difference between the minimum and themaximum scattering illumination intensity that is less than 30% of themaximum scattering illumination intensity.